fix string constants not being displayed correctly in the constants tab

[rerpha]

Description of work

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Ticket

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Acceptance criteria

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Unit tests

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System tests

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Documentation

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Code Review

• Is the code of an acceptable quality?
• If the change is to an OPI, does the check_opi_format.py script in C:\Instrument\Dev\ibex_gui\base\uk.ac.stfc.isis.ibex.opis pass?
• Do the changes function as described and is it robust?
• Is there associated PR for the release notes?

Final Steps

• Reviewer has also merged the release notes